The present invention relates to a method for forming electrochemical cells, including forming electrical connections within such cells.
Electrode materials formed by a transparent glass or plastic material substrate coated with a thin transparent layer of conducting material are generally referred to as transparent conductors, conducting glasses or conducting plastics. For reasons of convenience, the term xe2x80x9cconducting glassxe2x80x9d will be used throughout the present application to indicate both glass and plastic substrates being provided with a conducting layer. It should also be noted that the present invention is not limited to transparent conducting glass, but is useful also in cases wherein the substrate and/or the thin conducting layer is opaque.
Furthermore, for reasons of convenience the term xe2x80x9celectrochemical cellxe2x80x9d will be used throughout the present invention to designate xe2x80x9celectrochemical and/or photoelectrochemical cellxe2x80x9d, such as solar cells, cells in liquid crystal displays and battery cells.
The conducting layer is typically a few tenths of a micrometer thick. It consists typically of fluorine or indium doped tin oxide (ITO) or aluminum doped zinc oxide. Today there exist materials that exhibit approximately 80% light transparency within the visible range. The resistivity of these materials is typically approximately 5 ohm/cm2 or higher.
The electrical and optical properties of the conducting layer are inversely correlated to each other. Thus, in order to achieve a high conductivity a comparatively thick conducting layer has to be applied resulting in reduced light transparency. On the other hand, to achieve a good transparency a thin conducting layer should be applied resulting in reduced electrical conductivity.
Consequently, a compromise between transparency and electrical conductivity has to be accepted.
However, in certain applications, such as in solar cells, both high light transparency and high electrical conductivity are desired. Conventionally, this is achieved by the use of glass or plastic substrates having relatively high transparency. On these substrates numerous smaller separate solar cells are formed. Short circuit between separate solar cell units on the substrate is avoided by removing areas of the conducting layer between the cell segments. In order to avoid series resistance of the transparent conducting layers, current collectors are connected to each cell segment to conduct the current away from each individual cell.
The current collectors are formed from a material having high conductivity, such as metals or highly doped metal oxides, like those mentioned above.
To further minimize the internal resistive losses in a solar panel a high cell potential and low current is preferred. This is achieved by connecting in series a number of cells to a solar cell module so that the electrical potential from the entire solar cell module is increased.
In some electrochemical cells, such as the solar cells described in U.S. Pat. No. 5,525,440 to Kay et al., substances that act corrosively on the current collectors are present. In such cells the series connection devices must also withstand chemical deterioration. Generally, the current collectors should not corrode when the solar cells are stored in normal indoors or outdoors environment.
The methods used today to provide electrical connecting means in conducting layers of electrochemical cells include:
1) Deposition by evaporation. Using this method, the electrical connecting means are formed by depositing strings of highly conductive materials from vapor of, for example, metals or highly doped metal oxides. As the melting temperature of the evaporated material often is high, the depositing process has to be carried out at high temperature and vacuum conditions. The process equipment requires a lot of space. The method requires that a mask is placed in front of the substrate.
2) Sputtering. Also with the sputtering method the electrical connecting means are deposited on the substrate at a high temperature and in a vacuum. The process equipment requires a lot of space and high investment costs. Also the sputtering method requires that a mask is placed in front of the substrate.
(3) Screen printing, spraying etc. Solutions of small metal or conductive metal oxide particles are applied at a low temperature using conventional methods from the printing industry. The conductive particles are sintered together at high temperature to obtain good conductivity. An alternative is to use adhesives containing conductive particles such as silver conducting paint. Also with these methods it is required that a mask is placed in front of the substrate.
(4) Electroplating. The electrical connecting means can be applied to the conducting layer by electroplating. By immersing the substrate with the conducting layer in metal salt solutions, and then applying a negative potential on the conducting layer, metal is deposited directly on the conducting layer. Also with this method it is required that a mask is placed in front of the substrate.
Through U.S. Pat. No. 4,260,429 it is known to use metal wires as top electrodes by coating them with a solid polymer containing electrically conductive particles and then attach them to a semiconductor material by the application of heat and/or pressure. U.S. Pat. No. 4,260,429 teaches that the conductive particles are essential for the invention, since omitting them would result in a much reduced power output.
Through U.S. Pat. No. 5,084,107 a solar cell is known which includes a plated metal wire fixed to a light incident surface with an electrically conductive adhesive. U.S. Pat. No. 5,151,373 discloses a method for forming a solar cell similar to the solar cell of U.S. Pat. No. 5,084,107.
Through EP 0 807 980 A2 it is known to attach electrical wires, serving as current collectors, to a solar cell material in a solid state solar cell using an electrically conducting adhesive. The wires are attached to a supporting polymer film at the edge outside the cell using an adhesive, and a conducting bus bar is connected to the wires, for example using a conducting adhesive.
However, there still remains a need for a fast and cost effective method to produce electrochemical cells that provides electrical connection of conducting parts as well as chemical protection of the electrical connections.
In a first aspect, it is an object of the present invention to provide a novel method for forming electrochemical cells that is fast and provides electrical connection and chemical protection of the electrical connections in the same operative step.
This object is achieved by a method as defined in claim 1 of the appended claims.
The method of the invention provides advantages, as compared to previously known methods described above, such as no or reduced need for high temperature or vacuum environment, no need for masking and comparatively inexpensive process equipment. Furthermore, according to the invention a chemical protection of the wire, as well as a separating barrier between adjacent cells in a multi-cell assembly, is provided at the same time as the electrical connection is established.
In a second aspect, the object of the present invention is to provide an electrochemical cell assembly. This object is achieved by an assembly as defined in claim 7 of the appended claims.